(a) Fig. 1.1 shows a cross-section through an earth dam built on a farm. Fig. 1.1 (i) State the purpose of the spillway shown in the diagram. [2] (ii) Expla...

Labelled answer diagram:

(a)(i) The spillway allows excess water to overflow safely from the dam during periods of heavy rainfall. [1] Without a spillway, the water level could rise above the dam wall (overtop it), causing the earth to erode and potentially washing the entire wall away. [1] The spillway therefore acts as a controlled overflow channel that protects the structural integrity of the dam.

(a)(ii) The upstream face is lined with stones or concrete to protect the exposed earth from wave action and erosion caused by wind-driven water movement. [1] The lining also prevents water from seeping directly through the upstream surface of the wall, which would progressively soften and weaken the earth structure from within. [1]

(b) Three factors a farmer should consider when choosing a dam site (any three, 2 marks each): [6]

1. Topography: The site should be a narrow valley or gorge. A narrow cross-section reduces the length of dam wall needed, lowering construction costs and making the wall stronger relative to its length. [2]
2. Catchment size: The upstream catchment area must be large enough to collect sufficient rainfall runoff to fill the dam during the rainy season. A small catchment in a low-rainfall area will not generate enough inflow. [2]
3. Soil type: The soil at the site should be clay or have a high clay content. Clay particles are very small and pack tightly, forming an impermeable layer that holds water and prevents seepage losses through the dam floor. [2]

Other valid factors: the site should be above the area where water is needed (allowing gravity-fed distribution without pumps); the site should be away from heavy livestock traffic to reduce siltation and contamination.

(c) Two annual maintenance tasks (any two): [2]

1. Clear silt and sediment from the dam basin to maintain water storage capacity. [1]
2. Inspect and repair the spillway and dam wall for cracks or erosion damage. [1]

Other acceptable tasks include removing alien vegetation and reeds that increase water loss through transpiration, and checking fencing around the dam to keep livestock away from the wall.

(d) One disadvantage of relying on a single earth dam: During a drought the dam may dry up completely, leaving the herd with no water at all. [1] A single source is also vulnerable to contamination from a single pollution event or may lose capacity over time through siltation.